Optical pickup device having chromatic aberration correction lens

ABSTRACT

An optical pickup device includes a light source to emit light, an objective lens to focus the light on a recording medium to form a light spot, an optical path changer on an optical path between the light source and the objective lens to change the path of incident light, a chromatic aberration correction lens disposed on an optical path between the light source and the objective lens, and a photodetector to receive light which is reflected from the recording medium and is then incident thereon through the optical path changer. The chromatic aberration correction lens corrects a chromatic aberration occurring due to a change in the wavelength and/or due to an increase in a wavelength bandwidth of the light. The chromatic aberration correction lens includes at least two lenses such that a lens having a positive power and a lens having a negative power are adjacent to each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Patent Application No.2000-55477, filed Sep. 21, 2000, in the Korean Industrial PropertyOffice, the disclosure of which is incorporated herein by reference.

This application is a divisional of application Ser. No. 09/883,492filed on Jun. 19, 2001, now pending. The content of application Ser. No.09/883,492 is hereby incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical pickup device, and moreparticularly, to an optical pickup device having a chromatic aberrationcorrection lens to correct a chromatic aberration caused by a change ina wavelength and/or an increase in a wavelength bandwidth of lightemitted from a light source, occurring when changing arecording/reproducing power output.

2. Description of the Related Art

The recording capacity of an optical recording and reproducing apparatusis determined by the size S of a light spot formed on an optical disc bythe objective lens of an optical pickup device. Generally, the size S ofthe light spot is proportional to a wavelength λ and is inverselyproportional to a numerical aperture (NA). Accordingly, to obtain ahigher information recording density than that obtained on conventionaloptical discs such as CDs or DVDs, an optical pickup device(hereinafter, referred to as a high density optical pickup device) usedfor next generation DVDs (hereinafter, referred to as HD-DVDs) underdevelopment is anticipated to use a light source emitting blue light andan objective lens having a NA of at least 0.6, to reduce the size of thelight spot formed on the optical disc.

However, an optical material such as glass or plastic used as thematerial of the objective lens in the conventional optical pickup devicehas a very steep change in refractivity in a wavelength band shorterthan 650 nm. Table 1 shows changes in refractivity of M-BaCD5N, which ismanufactured by Hoya and is used as a glass material for molding theobjective lens, according to a wavelength. TABLE 1 Change inrefractivity of M-BaCD5N Change in wavelength glass manufactured by Hoya650 nm → 651 nm 0.000038 405 nm → 406 nm 0.000154

As seen from Table 1, an optical material has a change in refractivitywith respect to a small wavelength change of about 1 nm in a short bluewavelength band, for example, a 405 nm wavelength band, four timeslarger than in a 650 nm wavelength used in a conventional DVD opticalpickup device. Such a steep change in refractivity of the opticalmaterial with respect to blue light causes a high density opticalrecording and reproducing apparatus using a blue light source to bedefocused, thereby degrading performance.

In other words, an optical recording and reproducing apparatus usesdifferent recording light power and reproducing light power. This changein the light output power between recording and reproduction causes thewavelength change. For example, in the case of the blue light source,the change in the wavelength is about 0.5-1 nm. Usually, when the outputof the light source increases, the wavelength of light emitted from thelight source is longer. Accordingly, the high density optical pickupdevice using blue light has a large chromatic aberration in theobjective lens designed for a reference wavelength due to the change inthe wavelength during switching between recording light output power andreproducing light output power, causing defocus.

For example, as shown in FIGS. 1 through 3, an objective lens, which hasa numerical aperture of 0.65 and is designed for a wavelength of 405 nm,has a large wavefront aberration (also referred to as an optical pathdifference (OPD)) and defocus with respect to a fine change of about 1nm in wavelength. FIG. 1 is a graph illustrating intensities of lightspots formed on an optical disc according to defocus resulting from achange in light output power between recording and reproduction. FIGS. 2and 3 are graphs illustrating the amount of the OPD and the amount ofdefocus, respectively, of the objective lens having a numerical apertureof 0.65, according to the change in the wavelength.

Although defocus caused by the change in the wavelength can be correctedby adjusting the objective lens, it takes a relatively long time toactuate the objective lens using an actuator and to follow the change inthe wavelength, and during this time, the quality of a recorded orreproduced signal is degraded. Defocus occurring when output powerincreases for recording results in a lack of recording light power, anddefocus occurring when output power decreases for reproduction increasesjitter.

In other words, when the output power of the light source increases whenrecording information on the optical disc, the wavelength of lightemitted from the light source is relatively long, for example, 406 nm,so that the light spot formed on the optical disc is defocused. Untilthe actuator is adjusted in response to the defocus, recording cannot beperformed. Then, when the output power of the light source decreases forreproduction, the wavelength of light emitted from the light source isrelatively short, for example, 405 nm. Since the actuator has beenadjusted with respect to the lengthened wavelength, the light spot isdefocused again. As shown in FIG. 4, the jitter increases in thereproduced signal due to defocus. FIG. 4 is a graph illustrating theamount of jitter in the reproduced signal according to the amount ofdefocus when the objective lens designed with respect to a referencewavelength of 405 nm and having a numerical aperture of 0.65 is used.

Moreover, when the light source is actuated at a high frequency (HF) toreduce feedback noise of the light source due to light reflected fromthe optical disc to the light source, a wavelength bandwidth of thelight source increases, resulting in chromatic aberration, and thischromatic aberration degrades the reproduced signal.

Accordingly, a high density recordable optical pickup device capable ofrecording and reproducing repeatedly is required to have an opticalsystem capable of suppressing or correcting chromatic aberrationresulting from a change in the wavelength of light emitted from thelight source due to the change in output power between recording andreproduction. Japanese Patent Publication No. hei 9-311271 discloses astructure employing a refraction/diffraction-monolithic-type objectivelens to correct chromatic aberration resulting from a change inwavelength. A conventional refraction/diffraction-monolithic-typeobjective lens is an aspheric lens whose surface receiving or emittinglight is aspheric. Diffraction patterns are integrally formed on thisaspheric surface so that a refractive lens and a diffraction lens areintegrated into a single lens.

The refraction/diffraction-monolithic-type objective lens is designed tosatisfy (1+V_(HOE)/V)(n₂−1)>0.572 when it is assumed that refractivitiesof the lens at a central wavelength λ₁, a minimum wavelength λ₂ and amaximum wavelength λ₃ of light emitted from a semiconductor laser aren₁, n₂ and n₃, and that the Abbe numbers of the refractive lens and thediffraction lens are V=(n₂−1)/(n₁−n₃) and V_(HOE)=λ₂(λ₁−λ₃),respectively. Accordingly, the conventionalrefraction/diffraction-monolithic-type objective lens has a numericalaperture of at least 0.7 and can remove chromatic aberration due to thechange in the wavelength of light emitted from the semiconductor laser.However, an optical pickup device employing the conventionalrefraction/diffraction-monolithic-type objective lens cannot obtainsufficient output power necessary for recording since optical efficiencyis lowered to about 70-85% due to the properties of the diffractionlens.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide anoptical pickup device to correct a chromatic aberration of an objectivelens with an additional chromatic aberration correction lens having arelatively infinite focal length as compared to a focal length of theobjective lens.

It is a further object of the invention to provide an optical pickupdevice to overcome the above-mentioned problems.

Additional objects and advantages of the invention will be set forth inpart in the description which follows, and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

The foregoing objects of the present invention are achieved by providingan optical pickup device including a light source to emit light; anobjective lens to focus light incident from the light source on arecording medium to form a light spot; an optical path changer disposedon an optical path between the light source and the objective lens, theoptical path changer to change the path of light incident from therecording medium; a chromatic aberration correction lens disposed on theoptical path between the light source and the objective lens, thechromatic aberration correction lens to correct a chromatic aberrationoccurring due to a change in a wavelength and/or due to an increase in awavelength bandwidth of the light emitted from the light source, thechromatic aberration correction lens including a lens having a positivepower and a lens having a negative power adjacent to each other, a totalfocal length of the chromatic aberration correction lens beingrelatively infinite relative to the objective lens; and a photodetectorto receive light incident from the optical path changer.

The chromatic aberration correction lens has a focal length of at least10 m. Furthermore, the Abbe number of an optical material of which thelens having the positive power is formed, at a d-line, is larger thanthat of an optical material of which the lens having the negative poweris formed, at the d-line.

In one embodiment, the chromatic aberration correction lens includes afirst lens having a negative power and a second lens having a positivepower, which are sequentially disposed from the light source, and thefirst and second lenses have similar power. Here, the first and secondlenses are formed of glass materials, which have different Abbe numbersat a d-line and similar refractivities. The surfaces of the first andsecond lenses facing the light source and the objective lens,respectively, have relatively large negative radii of curvature, and thesurface between the first and second lenses has a relatively smallpositive radius of curvature.

In another embodiment, the chromatic aberration correction lens includesa first lens having a positive power and a second lens having a negativepower, which are sequentially disposed from the light source, thesurfaces of the first and second lenses facing the light source and theobjective lens, respectively, have positive radii of curvature, thesurface between the first and second lenses has a negative radius ofcurvature, and all the surfaces have similar magnitudes of radii ofcurvature.

In still another embodiment, the chromatic aberration correction lensincludes a first lens having a negative power, a second lens having apositive power and a third lens having a negative power, which aresequentially disposed from the light source. The first and third lensesare formed of glass materials, respectively, which have similar Abbenumbers at a d-line, and the second lens is formed of a glass materialhaving an Abbe number relatively different from those of the glassmaterials of the first and third lenses. The surfaces of the first andthird lenses facing the light source and the objective lens,respectively, have positive radii of curvature, the surface between thefirst and second lenses has a positive radius of curvature, and thesurface between the second and third lenses has a negative radius ofcurvature.

Here, preferably, the chromatic aberration correction lens is designedto satisfy 0.95≦h_(o)/h_(i)≦1.05, wherein a height of the light incidenton the chromatic aberration correction lens is h_(i), and the height oflight coming out through the chromatic aberration correction lens ish_(o). The chromatic aberration correction lens is designed to satisfy0<1/(f1·v1)+1/(f2·v2)+ . . . +1/(fn·vn)<0.008, wherein the focal lengthsof lenses constituting the chromatic aberration correction lens and theobjective lens with respect to the light source are f1, f2, . . . andfn, and the Abbe numbers of optical materials forming the lenses at ad-line are v1, v2, . . . and vn.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe preferred embodiments, taken in conjunction with the accompanyingdrawings of which:

FIG. 1 is a graph illustrating the intensities of light spots formed onan optical disc according to defocus resulting from a change in lightoutput power between recording and reproduction;

FIGS. 2 and 3 are graphs illustrating the amount of the wavefrontaberrations (or optical path difference (OPD)) and the amount ofdefocus, respectively, of an objective lens having a numerical apertureof 0.65, according to a change in a wavelength;

FIG. 4 is a graph illustrating the amount of jitter in a reproducedsignal according to the amount of defocus when an objective lensdesigned with respect to a reference wavelength of 405 nm and having anumerical aperture of 0.65 is used;

FIG. 5 is a schematic diagram illustrating the optical configuration ofa high density optical pickup device according to an embodiment of thepresent invention;

FIG. 6 is a schematic diagram illustrating the structure of an objectivelens having a numerical aperture of 0.75 with respect to a referencewavelength of 405 nm and the main optical paths thereof, when achromatic aberration correction lens according to the present inventionis not used;

FIG. 7 is a graph illustrating aberrations of the objective lens of FIG.6;

FIG. 8 is a schematic diagram illustrating the main portions and opticalpaths of an optical pickup device to which a chromatic aberrationcorrection lens according to a first embodiment of the present inventionis applied;

FIG. 9 is a graph illustrating aberrations of an objective lens in theoptical pickup device of FIG. 8;

FIG. 10 is a schematic diagram illustrating the main portions andoptical paths of an optical pickup device to which a chromaticaberration correction lens according to a second embodiment of thepresent invention is applied;

FIG. 11 is a graph illustrating aberrations of an objective lens in theoptical pickup device of FIG. 10;

FIG. 12 is a schematic diagram illustrating the main portions andoptical paths of an optical pickup device to which a chromaticaberration correction lens according to a third embodiment of thepresent invention is applied;

FIG. 13 is a graph illustrating aberrations of an objective lens in theoptical pickup device of FIG. 12;

FIG. 14 is a schematic diagram illustrating the main portions andoptical paths of an optical pickup device to which a chromaticaberration correction lens according to a fourth embodiment of thepresent invention is applied; and

FIG. 15 is a graph illustrating aberrations of an objective lens in theoptical pickup device of FIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the present invention, examples of which are illustratedin the accompanying drawings, wherein like reference numerals refer tolike elements throughout.

Referring to FIG. 5, an optical pickup device 105 according to anembodiment of the present invention includes a light source 10, anoptical path changing unit 100 to change an optical path of incidentlight, an objective lens 60 to focus incident light from the lightsource 10 on a high density recording medium 1 to form a light spot (notshown) thereon, a photodetector 90 to receive incident light, which isreflected from the recording medium 1 and passes through the opticalpath changing unit 100, and a chromatic aberration correction lens 40 tocorrect a chromatic aberration due to a change in a wavelength of thelight emitted from the light source 10 and an increase in a wavelengthbandwidth.

A blue semiconductor laser emitting light of a wavelength of at most 420nm, for example, a wavelength of 405 nm is used as the light source 10.The semiconductor laser may be either an edge emitting laser or avertical cavity surface emitting laser. Here, when the light source 10emits light having a wavelength of 405 nm at reproduction power, thelight source 10 emits light having a wavelength which is longer than thewavelength at the reproduction power, for example, a wavelength of 406nm, at recording power. Due to such a change in the wavelength resultingfrom a change in light output power and/or an increase in the wavelengthcaused by driving the light source 10 with high frequency (HF), achromatic aberration occurs in the objective lens 60. This chromaticaberration is corrected by the chromatic aberration correction lens 40according to the present invention as will be described later.

The optical path changing unit 100 is disposed between the light source10 and the objective lens 60 to change the path of incident light. Asshown in FIG. 5, the optical path changing unit 100 preferably includesa polarizing beam splitter 50 to selectively transmit or reflectincident light according to a polarization characteristic of theincident light, and a quarter wavelength plate 55 to change thepolarization of the incident light. Here, a beam splitter (not shown),which transmits and reflects incident light at a predetermined ratio,can be used as the optical path changing unit 100.

The objective lens 60 has a numerical aperture of at least 0.65, forexample, 0.75 or 0.85, so that it can form the light spot on the highdensity recording medium 1, which may be an HD-DVD to record andreproduce information. Here, the objective lens 60 may have a numericalaperture of at least 0.85 when it is composed of a plurality of lensesor is of a solid immersion type. The photodetector 90 receives lightreflected from the recording medium 1 and detects an information signaland an error signal.

A collimating lens 20 is disposed on the optical path between the lightsource 10 and the chromatic aberration correction lens 40. Thecollimating lens 20 condenses diverging light emitted from the lightsource 10 to be parallel. As shown in FIG. 5, when the collimating lens20 is disposed on the optical path between the light source 10 and theoptical path changing unit 100, a condensing lens 70 is also disposedbetween the optical path changing unit 100 and the photodetector 90.

When an edge emitting laser is used as the light source 10, a beamshaping prism 30 is disposed on the optical path between the collimatinglens 20 and the optical path changing unit 100 so that recording ofinformation is possible even with low power. Although not shown in FIG.5, the beam shaping prism 30 shapes an elliptical-like beam emitted fromthe edge emitting laser into a circular-like beam. The beam shapingprism 30 may alternately be disposed between the light source 10 and thecollimating lens 20. As another alternative, when a surface emittinglaser emitting a substantially circular-like beam is used as the lightsource 10, the beam shaping prism 30 can be removed from the pickupdevice 105 of FIG. 5.

Here, reference numeral 80 denotes a sensing lens 80. For example, whena focus error signal is detected by an astigmatism method, the sensinglens 80 is an astigmatism lens to include an astigmatism into theincident light.

The chromatic aberration correction lens 40 according to the presentinvention comprises at least two lenses such that a lens having apositive power and a lens having a negative power are disposed to beadjacent to each other. Here, the Abbe number of an optical material, ofwhich the lens having the positive power is formed, at a d-line, exceedsthat of an optical material, of which the lens having the negative poweris formed, at the d-line.

When the focal lengths of the lenses with respect to the light source 10are f1, f2, . . . and the Abbe numbers of the optical materials formingthe lenses at the d-line are v1, v2, . . . , a condition to correct thechromatic aberration is usually expressed by${\sum\limits_{i}\frac{1}{{f_{i} \cdot \upsilon}\quad i}} = 0.$Considering this condition, the chromatic aberration correction lens 40according to the present invention is designed, as will be describedlater in detailed embodiments, such that it satisfies the condition that$\sum\limits_{i}\frac{1}{{f_{i} \cdot \upsilon}\quad i}$is approximately 0, that is, it satisfies a range given by Equation (1),thereby effectively correcting the chromatic aberration of the objectivelens 60. $\begin{matrix}{0 < {\sum\limits_{i}\frac{1}{{f_{i} \cdot \upsilon}\quad i}} < 0.008} & (1)\end{matrix}$

When the optical pickup device 105 according to the present inventionincludes the collimating lens 20, as shown in FIG. 5, so that parallellight is incident on the chromatic aberration correction lens 40, lensescontributing to $\sum\limits_{i}\frac{1}{{f_{i} \cdot \upsilon}\quad i}$indicating the correction degree of the chromatic aberration are thechromatic aberration correction lens 40 and the objective lens 60.

The chromatic aberration correction lens 40 according to the presentinvention as described above has a relatively infinite focal length, forexample, a focal length of at least 10 m, as compared with the objectivelens 60, so that it has optical power close to 0.

Hereinafter, detailed embodiments of the chromatic aberration correctionlens 40 according to the present invention and the optical design datafor the objective lens 60 and the chromatic aberration correction lens40 will be described in detail. In the following embodiments, an opticalpickup device according to the present invention includes thecollimating lens 20 so that parallel light is incident on the chromaticaberration correction lens 40 or on the objective lens 60, and opticaldata suitable for a reference wavelength of 405 nm is used as anexample.

First, in the case where the chromatic aberration correction lens 40according to the present invention is not used, the degree of aberrationoccurring in the objective lens 60 is observed when the wavelength oflight emitted from the light source 10 changes from the referencewavelength of 405 nm into a wavelength of 406 nm. When the objectivelens 60 has a numerical aperture of 0.75 with respect to the referencewavelength of 405 nm, referring to FIG. 6 and Table 2, the objectivelens 60 is realized as a bi-convex lens whose both surfaces are asphericso that the objective lens 60 focuses incident parallel light on therecording medium 1 having a thickness of 0.6 mm to form a light spotthereon. TABLE 2 Radius of Gap or Abbe curvature thick- Material numberat Element (mm) ness (mm) (glass) Refractivity the d-line Objective 2.012300 1.700000 ‘OG’ 1.623855 57.8 lens 60 (aspheric surface 1)−18.075156 1.656000 (aspheric surface 2) Recording ∞ 0.600000 ‘CG’1.621462 31.0 medium 1

Table 3 shows the conic constants and aspheric coefficients of theaspheric surfaces 1 and 2 of the objective lens 60. TABLE 3 Conicconstants (K) Aspheric coefficients Aspheric −0.928355 A: 0.737867E−02B: 0.515008E−03 surface 1 C: 0.109070E−03 D: −0.961470E−04 E:0.755098E−04 F: −0.342032E−04 G: 0.921692E−05 H: −0.137595E−05 J:0.843459E−07 Aspheric −135.791497 A: 0.864934E−02 B: −0.203022E−02surface 2 C: 0.375653E−03 D: −0.431759E−04 E: −0.337619E−05 F:−0.123502E−06 G: 0.142911E−06 H: 0.433818E−07 J: −0.410333E−08

Here, when a depth from the apex of an aspheric surface is representedby “z”, the $\begin{matrix}{z = {\frac{c\quad h^{2}}{1 + \sqrt{1 - {\left( {1 + K} \right)c^{2}h^{2}}}} + {A\quad h^{4}} + {B\quad h^{6}} + {C\quad h^{8}} + {D\quad h^{10}} + {E\quad h^{12}} + {F\quad h^{14}} + {G\quad h^{16}} + {H\quad h^{18}} + {J\quad h^{20}}}} & (2)\end{matrix}$depth z can be expressed by Equation (2).

Here, h is a height from an optical axis, c is a curvature, K is a coniccoefficient, and A through J are aspheric coefficients.

The diameter of an incident pupil of parallel light on the objectivelens 60 configured as described above is 3.9 mm, and the focal length ofthe objective lens 60 is about 3.0000 mm.

FIG. 7 shows the degrees of aberration of the objective lens 60 of FIG.6. As shown in FIG. 7, a large aberration occurs in the objective lens60 when the wavelength of light emitted from the light source 10 changesfrom 405 nm, i.e., the reference wavelength, to 406 nm. However,aberration occurring in the objective lens 60 is removed by a chromaticaberration correction lens 40, 140 or 240 installed to the side of theincident pupil of the objective lens 60, according to the presentinvention, as described below.

FIGS. 8, 10 and 12 show the chromatic aberration correction lenses 40,140 and 240 according to embodiments of the present invention, which areinstalled to the side of the incident pupil of the objective lens 60described with reference to FIG. 6. Tables 4 through 6 show the opticaldesign data of the chromatic aberration correction lenses 40, 140 and240 and the objective lens 60. In Tables 4 through 6, the objective lens60 has a numerical aperture of 0.75 with respect to the referencewavelength of 405 nm, and the optical design data thereof is the same asshown in Table 2. In addition, the conic constants and asphericcoefficients of the aspheric surfaces 1 and 2 of the objective lens 60are the same as those shown in Table 3, and the focal length thereof is3.000 mm. Each of the chromatic aberration correction lenses 40, 140 and240 according to the embodiments of the present invention is configuredsuch that at least two lenses having opposite powers are adjacent toeach other. Among the at least two lenses, a lens having a positivepower is formed of an optical material whose Abbe number at the d-lineis larger than that of a material of which a lens having a negativepower is formed.

Referring to FIG. 8 and Table 4, the chromatic aberration correctionlens 40 according to a first embodiment of the present invention iscomprised of a first lens 41 having a negative power and a second lens45 having a positive power, which are sequentially disposed from thelight source 10. The first and second lenses 41 and 45 have almost thesame power magnitude. As shown in Table 4, the first and second lenses41 and 45 are formed of glass materials having similar refractivitiesand different Abbe numbers at the d-line. The surfaces S1, S3 of thefirst and second lenses 41 and 45 facing the light source 10 and theobjective lens 60, respectively, have relatively large radii ofcurvature, and the contact surface S2 between the first and secondlenses 41 and 45 has a smaller radius of curvature. TABLE 4 Radius ofAbbe curvature Thickness/ number at Element Surfaces (mm) gap (mm)Material Refractivity the d-line Chromatic S1 −51.340719 1.000000 EFD151.741876 30.1 aberration S2 3.000000 2.300000 LAF3 1.742841 48.0correction S3 −53.981665 10.00000 lens 40 Objective S4 2.012300 1.700000‘OG’ 1.623855 57.8 lens 60 (aspheric surface 1) S5 −18.075156 1.656000(aspheric surface 2) Recording S6 ∞ 0.600000 ‘CG’ 1.621462 31.0 medium 1

In the chromatic aberration correction lens 40 having the abovestructure according to the first embodiment of the present invention,the focal length of the first lens 41 is −3.790843 mm, the focal lengthof the second lens 45 is 3.892900 mm, and the total focal length of thechromatic aberration correction lens 40 is about 171.985311426 m. Theincident pupil diameter of the objective lens 60 is 3.9 mm. According tothe chromatic aberration correction lens 40 and the objective lens 60having the optical design data shown in Table 4,$\sum\limits_{i}\frac{1}{{f_{i} \cdot \upsilon}\quad i}$approximates to 0, that is,${\sum\limits_{i}\frac{1}{{f_{i} \cdot \upsilon}\quad i}} \cong 0.0024$Therefore, the chromatic aberration occurring in the objective lens 60due to a change in the wavelength of light emitted from the light source10 when the chromatic aberration correction lens 40 is not used, asshown in FIG. 7, can be removed by employing the chromatic aberrationcorrection lens 40 according to the first embodiment of the presentinvention. Consequently, in the case where the optical system structureof FIG. 8 and the optical design data shown in Table 4 are provided,referring to FIG. 9 illustrating the degrees of aberration of theobjective lens 60, aberration rarely occurs in the objective lens 60even when the wavelength of light emitted from the light source 10changes from 405 nm, that is, the reference wavelength, to 406 nm.

Referring to FIG. 10 and Table 5, the chromatic aberration correctionlens 140 according to a second embodiment of the present invention iscomprised of a first lens 141 having a positive power and a second lens145 having a negative power, which are sequentially disposed from thelight source 10. As shown in Table 5, the surfaces S1, S3 of the firstand second lenses 141 and 145 facing the light source 10 and theobjective lens 60, respectively, have positive radii of curvature, andthe contact surface S2 between the first and second lenses 141 and 145has a negative radius of curvature. The magnitudes of the radii ofcurvature of the surfaces S1, S2, S3 of the first and second lenses 141and 145 are similar to one another. TABLE 5 Radius of Abbe curvatureThickness/ number at Element Surfaces (mm) gap (mm) MaterialRefractivity the d-line Chromatic S1 7.320225 2.300000 LAFL2 1.72176648.5 aberration S2 −6.459849 1.000000 EFD15 1.741876 30.1 correction S36.292012 10.00000 lens 140 Objective S4 2.012300 1.700000 ‘OG’ 1.62385557.8 lens 60 (aspheric surface 1) S5 −18.075156 1.656000 (asphericsurface 2) Recording S6 ∞ 0.600000 ‘CG’ 1.621462 31.0 medium 1

In the chromatic aberration correction lens 140 having the abovestructure according to the second embodiment of the present invention,the focal length of the first lens 141 is 5.112121 mm, the focal lengthof the second lens 145 is −4.157561 mm, and the total focal length ofthe chromatic aberration correction lens 140 is about 109.823479554 m.The incident pupil diameter of the objective lens 60 is 4.8 mm.According to the chromatic aberration correction lens 140 and theobjective lens 60 having the optical design data shown in Table 5,$\sum\limits_{i}\frac{1}{{f_{i} \cdot \upsilon}\quad i}$approximates to 0, that is,${\sum\limits_{i}\frac{1}{{f_{i} \cdot \upsilon}\quad i}} \cong {0.0019.}$Consequently, in the case where the optical system structure of FIG. 10and the optical design data shown in Table 5 are provided, as shown inFIG. 11 illustrating the aberration of the objective lens 60, when thechromatic aberration correction lens 140 according to the secondembodiment of the present invention is used, the chromatic aberration iscorrected so that aberration is minimal in the objective lens 60 evenwhen the wavelength of light emitted from the light source 10 changesfrom 405 nm, that is, the reference wavelength, to 406 nm, similar tothe case of using the chromatic aberration correction lens 40 accordingto the first embodiment of the present invention.

Referring to FIG. 12 and Table 6, the chromatic aberration correctionlens 240 according to a third embodiment of the present invention iscomprised of a first lens 241 having a negative power, a second lens 243having a positive power and a third lens 245 having a negative power,which are sequentially disposed from the light source 10. As shown inTable 6, the first and third lenses 241 and 245 are formed of glassmaterials having similar Abbe numbers at the d-line, and the second lens243 is formed of a glass material having an Abbe number at the d-linewhich is different from those of the first and third lenses 241 and 245.The surfaces S1, S4 of the first and third lenses 241 and 245 facing thelight source 10 and the objective lens 60, respectively, have positiveradii of curvature, the surface S2 between the first and second lenses241 and 243 has a positive radius of curvature, and the surface S3between the second and third lenses 243 and 245 has a negative radius ofcurvature. TABLE 6 Radius of Abbe curvature Thickness/ number at ElementSurfaces (mm) gap (mm) Material Refractivity the d-line Chromatic S17.564520 1.000000 EFD4 1.806295 27.5 aberration S2 5.252096 3.000000BACD5 1.605256 61.3 correction S3 −11.863307 1.000000 EFD10 1.77591628.3 lens 240 S4 10.217745 10.00000 Objective S5 2.012300 1.700000 ‘OG’1.623855 57.8 lens 60 (aspheric surface 1) S6 −18.075156 1.656000(aspheric surface 2) Recording S7 ∞ 0.600000 ‘CG’ 1.621462 31.0 medium 1

In the chromatic aberration correction lens 240 having the abovestructure according to the third embodiment of the present invention,the focal length of the first lens 241 is 26.405720 mm, the focal lengthof the second lens 243 is 6.440303 mm, the focal length of the thirdlens 245 is −6.937722, and the total focal length of the chromaticaberration correction lens 240 is about 116.040546093 m. The incidentpupil diameter of the objective lens 60 is 5.0 mm. According to thechromatic aberration correction lens 240 and the objective lens 60having the optical design data shown in Table 6,$\sum\limits_{i}\frac{1}{{f_{i} \cdot \upsilon}\quad i}$approximates to 0, that is,${\sum\limits_{i}\frac{1}{f\quad{i \cdot \upsilon}\quad i}} \cong {0.0019.}$In other words, chromatic aberration occurring in the objective lens 60can be almost removed when the chromatic aberration correction lens 240according to this embodiment is used, similar to the case of using thechromatic aberration correction lens 40 according to the firstembodiment of the present invention. Consequently, in the case where theoptical system structure of FIG. 12 and the optical design data shown inTable 6 are provided, as shown in FIG. 13 illustrating the degrees ofaberration of the objective lens 60, when the chromatic aberrationcorrection lens 240 according to the third embodiment of the presentinvention is used, chromatic aberration is corrected so that aberrationis minimal in the objective lens 60. This holds true even when thewavelength of light emitted from the light source 10 changes from 405nm, that is, the reference wavelength, to 406 nm, like the case of usingthe chromatic aberration correction lens 40 according to the firstembodiment of the present invention.

The chromatic aberration correction lenses 40, 140 and 240 according tothe first through third embodiments of the present invention describedabove are designed to be suitable for a high density optical pickupdevice, which includes the objective lens 60 having a numerical apertureof 0.75 and is suitable for the recording medium 1 having a thickness of0.6 mm. Even if the numerical aperture of the objective lens 60 and thethickness of the recording medium 1 change, the chromatic aberration canbe effectively corrected as in the above three embodiments just byappropriately changing the optical design data of each of the chromaticaberration correction lenses 40, 140 and 240. In other words, when ahigh density optical pickup device according to the present invention isdesigned to form a light spot on a recording medium 1 having a thicknessof smaller than 0.6 mm with an objective lens 60 having a numericalaperture of larger than 0.75, each of the chromatic aberrationcorrection lenses 40, 140 and 240 having structures according to thefirst through third embodiments of the present invention is newlydesigned to be suitable for the conditions of the objective lens 60 andthe recording medium 1.

For example, when an optical pickup device 105 according to the presentinvention is designed such that an objective lens 60′ having a numericalaperture of 0.85 with respect to the reference wavelength of 405 nmfocuses incident parallel light on a recording medium 1′ having athickness of 0.1 mm to form a light spot, the optical structure and theoptical design data of the objective lens 60′ and the chromaticaberration correction lens 40 according to the first embodiment arechanged as shown in FIG. 14 and Table 7, resulting in chromaticaberration correction lens 340. TABLE 7 Radius of Abbe curvatureThickness/ number at Element Surfaces (mm) gap (mm) MaterialRefractivity the d-line Chromatic S1 −1114.82920 1.000000 EFD15 1.74187630.1 aberration S2 2.57236 3.000000 LAF3 1.742841 48.0 correction S3−2735.69376 10.00000 lens 340 Objective S4 1.41052 2.750000 ‘OG’1.715566 53.2 lens 60′ (aspheric surface 1′) S5 −2.48758 0.271251(aspheric surface 2′) Recording S6 ∞ 0.100000 ‘CG’ 1.621462 31.0 medium1′

The objective lens 60′ is a bi-convex lens whose both surfaces areaspheric. Table 8 shows the conic constants and aspheric coefficients ofthe aspheric surfaces S4 and S5 of the objective lens 60′. TABLE 8 Conicconstants (K) Aspheric coefficients Aspheric −0.697423 A: 0.121877E−01B: 0.186663E−02 surface S4 C: 0.411872E−03 D: −0.145635E−03 E:0.658968E−04 F: 0.224260E−04 G: 0.560839E−05 H: −0.307800E−05 J:−0.233787E−05 Aspheric −27.258190 A: 0.359235E+00 B: 0.784442E−01surface S5 C: −0.172135E+01 D: 0.196996E+01 E: −0.111915E−09 F:−0.913659E−11 G: −0.735287E−12 H: −0.175404E−13 J: 0.636830E−15

The incident pupil diameter of light incident on the objective lens 60′in parallel is 3.03 mm, and the focal length of the objective lens 60′is about 1.782400 mm.

Like the chromatic aberration correction lens 40 according to the firstembodiment of the present invention described above with reference toFIG. 8 and Table 4, the chromatic aberration correction lens 340 iscomprised of a first lens 341 having a negative power and a second lens345 having a positive power, which are sequentially disposed from thelight source 10. As shown in Table 7, the first and second lenses 341and 345 are formed of glass materials having similar refractivities anddifferent Abbe numbers at the d-line. The surfaces S1, S3 of the firstand second lenses 341 and 345 facing the light source 10 and theobjective lens 60′, respectively, have very large negative radii ofcurvature, and the surface S2 between the first and second lenses 341and 345 has a small radius of curvature.

When the chromatic aberration correction lens 340 having the abovestructure is configured to be suitable for the objective lens 60′ havinga numerical aperture of 0.85 and the recording medium 1′ having athickness of 0.1 mm based on the optical data shown in Table 7, thefocal length of the first lens 341 is −3.45806 mm, the focal length ofthe second lens 345 is 3.460852 mm, and the total focal length of thechromatic aberration correction lens 340 is about −53.801051977 m.According to the chromatic aberration correction lens 340 and theobjective lens 60′ having the optical design data shown in Tables 7 and8, $\sum\limits_{i}\frac{1}{f\quad{i \cdot \upsilon}\quad i}$approximates to 0, that is,${\sum\limits_{i}\frac{1}{f\quad{i \cdot \upsilon}\quad i}} \cong {0.0070.}$

FIG. 15 illustrates the aberration of the objective lens 60′ when theoptical system structure of FIG. 14 and the optical design data shown inTables 7 and 8 are provided. As shown in FIG. 15, even when thewavelength of light emitted from the light source 10 changes from 405nm, that is, the reference wavelength, to 406 nm, the chromaticaberration is corrected by the chromatic aberration correction lens 340so that aberration is minimal in the objective lens 60′. Accordingly,even when the chromatic aberration correction lens 340 according to thepresent invention is adopted for an ultrahigh density optical pickupdevice, for example, which forms a light spot on the recording medium 1′having a thickness of 0.1 mm with the objective lens 60′ having anumerical aperture of about 0.85, the chromatic aberration correctionlens 340 can effectively remove chromatic aberration occurring in theobjective lens 60′.

As is known from the above detailed embodiments, in a high densityoptical pickup device employing a chromatic aberration correction lensaccording to the present invention,$\sum\limits_{i}\frac{1}{f\quad{i \cdot \upsilon}\quad i}$has a value which is close to 0 and satisfies the range defined byEquation (1). In addition, a chromatic aberration correction lensaccording to the present invention has an optical power of nearly 0 andan infinite focal length of at least 10 m. Accordingly, when the heightof light incident on the chromatic aberration correction lens is h_(i),and the height of light coming out through the chromatic aberrationcorrection lens is h_(o), the chromatic aberration correction lenssatisfies 0.95≦h_(o)/h_(i)≦1.05. Consequently, a chromatic aberrationcorrection lens according to the present invention can correct chromaticaberration occurring in an objective lens due to a change in awavelength resulting from a change in the light output power of thelight source 10 and/ or due to an increase in a wavelength bandwidthcaused by driving the light source 10 with HF, and is advantageous inthat it can be simply added to an optical pickup device without changingthe optical system structure of the optical pickup device.

As described above, a high density optical pickup device according tothe present invention is provided with a chromatic aberration correctionlens having an infinite focal length as compared to an objective lensand corrects chromatic aberration using the refraction of opticalmaterials, thereby having a high light efficiency. In addition, anoptical pickup device according to the present invention is providedwith a collimating lens to change diverging light emitted from a lightsource into parallel light and a separate chromatic aberrationcorrection lens, thereby recording information with light of relativelylow power. Moreover, since a chromatic aberration correction lens has anoptical power of nearly 0, it can be simply installed without changingthe optical system structure.

Although a few preferred embodiments of the present invention have beenshown and described, it would be appreciated by those skilled in the artthat changes may be made in these embodiments without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. An optical pickup device comprising: a light source to emit light; anobjective lens to focus the emitted light from the light source on arecording medium to form a light spot; an optical path changer disposedon an optical path of the emitted light between the light source and theobjective lens, the optical path changer changing the optical path ofthe emitted light; a chromatic aberration correction lens disposed onthe optical path between the light source and the objective lens, thechromatic aberration correction lens correcting a chromatic aberrationoccurring due to a change in a wavelength of the emitted light and/ordue to an increase in a wavelength bandwidth of the emitted light, thechromatic aberration correction lens comprising a first lens having apositive power and a second lens having a negative power adjacent toeach other, a total focal length of the chromatic aberration correctionlens being infinite as compared to the objective lens; and aphotodetector to receive the light reflected from the recording mediumand then incident thereon through the optical path changer, wherein thefirst lens having the positive power and the second lens having thenegative power are sequentially disposed from the light source, surfacesof the first and second lenses facing the light source and the objectivelens, respectively, have positive radii of curvature, a surface betweenthe first and second lenses has a negative radius of curvature, and theradii of curvature of the surfaces facing the light source, theobjective lens and between the first and second lenses have similarmagnitudes.
 2. The optical pickup device of claim 1, wherein thechromatic aberration correction lens further comprises a third lenshaving a negative power, and the second lens having the negative power,the first lens having the positive power, and the third lens having thenegative power are sequentially disposed from the light source.
 3. Theoptical pickup device of claim 2, wherein the first and third lenses areformed of glass materials, respectively, which have similar Abbe numbersat a d-line, and the second lens is formed of a glass material having anAbbe number different from those of the glass materials of the first andthird lenses.
 4. The optical pickup device of claim 2, wherein surfacesof the second and third lenses facing the light source and the objectivelens, respectively, have positive radii of curvature, a surface betweenthe first and second lenses has a positive radius of curvature, and asurface between the first and third lenses has a negative radius ofcurvature.
 5. The optical pickup device of claim 1, wherein thechromatic aberration correction lens satisfies 0.95≦h_(o)/h_(i)≦1.05when a height of the emitted light incident on the chromatic aberrationcorrection lens is h_(i), and a height of light exiting the chromaticaberration correction lens is h_(o).
 6. The optical pickup device ofclaim 2, wherein the chromatic aberration correction lens satisfies0.95≦h_(o)/h_(i)≦1.05 when a height of the emitted light incident on thechromatic aberration correction lens is h_(i), and a height of theemitted light exiting the chromatic aberration correction lens is h_(o).7. The optical pickup device of claim 1, wherein the chromaticaberration correction lens satisfies 0<1/(f1·v1)+1/(f2·v2)+ . . .+1/(fn·vn)<0.008 when focal lengths of the lenses comprising thechromatic aberration correction lens and the objective lens with respectto the light source are f1, f2, . . . and fn, and Abbe numbers ofoptical materials forming the lenses comprising the chromatic aberrationcorrection lens and the objective lens at a d-line are v1, v2, . . . andvn.
 8. The optical pickup device of claim 2, wherein the chromaticaberration correction lens satisfies 0<1/(f1·v1)+1/(f2·v2)+ . . .+1/(fn·vn)<0.008 when focal lengths of the lenses comprising thechromatic aberration correction lens and the objective lens with respectto the light source are f1, f2, . . . and fn, and Abbe numbers ofoptical materials forming the lenses comprising the chromatic aberrationcorrection lens and the objective lens at a d-line are v1, v2, . . . andvn.